In the related art, a winding-type transformer has been widely used for measuring a current of a power system in power facilities. However, the winding-type transformer is greatly enlarged as a to-be-measured system voltage is increased, and thus, there is a problem in that costs and installation space are increased. Particularly, in a gas insulation switching apparatus using an insulating gas, which is called GIS (Gas Insulation Switch), since miniaturization and space saving are greatly required, it is difficult to install a large-sized winding-type transformer therein.
For this reason, in terms of miniaturization, space saving, high insulation property, and noise resistance, various current sensors which are configured to include an optical fibre installed to surround the current conductor and perform current measurement by using the Faraday effect of the optical fibre have been proposed and used in the related art. In the current sensor, a linearly polarized light beam is incident on the optical fibre; the optical fibre is configured to surround the conductor through which the to-be-measured current flows; and due to the Faraday effect of the optical fibre, a polarization plane of the linearly polarized light beam in the optical fibre is rotated by a magnetic field generated in proportion to a current. At this time, the rotation angle of the polarization plane is proportional to a magnitude of the to-be-measured current. Therefore, by measuring the rotation angle, the magnitude of the current can be obtained.
FIG. 15 is a schematic diagram illustrating a current sensor disclosed in Patent Literature 1 as an example of a current sensor using the Faraday effect of an optical fibre. The current sensor 100 is configured to include an optical circulator 101, a birefringent element 102, a Faraday rotator 103, and an optical fibre 104 for a sensor. The optical fibre 104 is disposed along the outer circumference of the conductor 105 through which a to-be-measured current flows. The Faraday rotator 103 is installed at the one end of the optical fibre 104, and a mirror 106 is installed at the other end thereof. In addition, the birefringent element 102 and the optical circulator 101 are connected to each other by the optical fibre. The optical circulator 101 is connected in such a direction that the light beam of the light source 107 transmits toward the optical fibre 104 side.
The light beam which is emitted from the light source 107 to be incident through the optical fibre 108 and the optical circulator 101 on the birefringent element 102 is converted into a linearly polarized light beam by the birefringent element 102 to be incident on the Faraday rotator 103. The Faraday rotator 103 is configured to include a magnet 109 and a ferromagnetic garnet 110 which is magnetically saturated by the magnet 109 so as to rotate the polarization plane of the light beam transmitting through the ferromagnetic garnet 110 by 22.5 degrees. The linearly polarized light beam transmitting through the Faraday rotator 103 is incident on the optical fibre 104 to be subject to Faraday rotation by a magnetic field generated by the to-be-measured current flowing through the conductor 105, so that the polarization plane of the linearly polarized light beam is rotated by a rotation angle which is proportional to the intensity of the magnetic field.
The light beam propagating the optical fibre 104 is further reflected by the mirror 106, and when the light beam propagates the optical fibre 104 again, the light beam is rotated due to the Faraday effect again by the magnetic field to be incident on the Faraday rotator 103 again. Since the light beam transmits through the Faraday rotator 103 again, the polarization plane is further rotated by 22.5 degrees, so that the polarization plane is rotated by 45 degrees in reciprocating paths by the Faraday rotator 103. The light beam transmitting through the Faraday rotator 103 propagates the birefringent element 102 again to be divided into two linearly polarized light beams of which the polarization directions are perpendicular to each other. The one divided linearly polarized light beam is received by a light-receiving element 112 through the optical circulator 101 and an optical fibre 111 to be converted into an electric signal S1. In addition, the other linearly polarized light beam is received by a light-receiving element 114 through an optical fibre 113 to be converted into an electric signal S2.
Since the light amount received by the light-receiving elements 112 and 114 is changed according to the Faraday rotation angle occurring in the linearly polarized light beam propagating the optical fibre 104, the electric signals S1 and S2 are processed by a signal processing circuit 115 by taking into consideration the change to obtain the Faraday rotation angle generated by the optical fibre 104. Next, the to-be-measured current is calculated from the obtained Faraday rotation angle.
In addition, since power transmission-transformation facilities such as the above-described GIS are configured to have a large magnitude of the current, in order to detect the large-magnitude current by the optical fibre, a quartz-based optical fibre having a large maximum measurement current value needs to be used.
However, in the case where the quartz-based optical fibre is used as the optical fibre 104 performing current detection, linear birefringence occurs due to a stress generated from bending or vibration, and thus the propagating linearly polarized light beam is converted to have an elliptic form, so that measurement error is increased. In other words, if external vibration is applied to a current sensor such as the current sensor 100 using the Faraday effect of the optical fibre, there is a problem in that the measurement result of the to-be-measured current is greatly changed due to photoelasticity of the optical fibre.
For example, FIG. 16 illustrates an example where the measurement result is changed according to operations of a breaker in the above-described GIS. In a steady-state case where a system frequency is 60 Hz, the measurement result is a waveform illustrated in FIG. 16(a). If vibration is applied in the steady state, the measurement result is greatly changed as illustrated in FIG. 16(b).
Therefore, a low birefringence optical fibre containing lead oxide is used as the optical fibre 104. The reason why the optical fibre containing lead oxide is used is that a photoelasticity coefficient thereof is much smaller than that of the quartz-based optical fibre and thus, the propagating polarized light beam is not easily influenced by the stress due to bending or vibration.
However, since Verdet constant indicating Faraday rotation ability of the optical fibre containing lead oxide is about five times Verdet constant of the quartz-based optical fibre, the maximum detection current thereof is smaller than that of the quartz-based optical fibre, so that the optical fibre containing lead oxide has disadvantage in measurement of a large-magnitude current.
Therefore, as means for suppressing the above-described problems occurring in the optical fibre, a method of replacing a mirror 106 with the Faraday mirror having the Faraday rotator and optically connecting the Faraday mirror to the other end of the optical fibre 104 is considered. For example, Patent Literature 2 discloses an example of the Faraday mirror having the Faraday rotator.
FIG. 18 illustrates a configuration of a Faraday mirror 123 disclosed in Patent Literature 2. The Faraday mirror 123 is configured by inserting an optical fibre 126, an optical fibre 127, and a converging beam series integrating terminal having a spherical portion 128 at the distal end into a central hole of an optical fibre holder 124 through a ferrule 125, allowing the spherical portion 128 to protrude, disposing a 45-degree Faraday rotator 129 and a mirror 130 to face each other, and being sealed by a cap 132 to externally enclosing a magnet 131 for magnetizing the Faraday rotator 129. If a direction in which a light beam propagates from the optical fibre 127 to the mirror 130 is defined as a forward direction and a direction in which a light beam propagates from the mirror 130 to the optical fibre 127 is defined as a backward direction. In the forward direction, a polarization plane of the light beam which propagates through the optical fibre 127 and is emitted from the spherical portion 128 is rotated by 45 degrees by the Faraday rotator 129 and is reflected by the mirror 130. In addition, in the backward direction, the light beam transmits through the Faraday rotator 129 again, so that the polarization plane thereof is further rotated by 45 degrees. Therefore, the light beam is returned to the optical fibre 127 in the state where the polarization plane in the backward direction is rotated by 90 degrees from the polarization plane of the light beam emitted from the optical fibre 127 and the spherical portion 128 in the forward direction.
However, although the vibration characteristic of the current sensor including the Faraday mirror 123 is improved in comparison with the vibration characteristic of the current sensor including the mirror 106, the vibration characteristic is not sufficient for highly accurate measurement, and a temperature characteristic thereof is deteriorated. The reason is as follows. The Faraday rotator 129 has a temperature characteristic and a wavelength characteristic; there is a limitation in the thickness processing accuracy for determining the Faraday rotation angle of 45 degrees; and at the temperature and wavelength during the measurement, when the light beam reciprocates the Faraday rotator, the Faraday rotation angle of the polarization plane thereof is shifted from 90 degrees, so that the birefringence of the optical fibre cannot be completely compensated. In addition, the wavelength and temperature characteristics of the measurement accuracy of the current sensor are also deteriorated. FIG. 19 illustrates temperature dependency of the measured value of the to-be-measured current output from the current sensor connected to the Faraday mirror 123 as a ratio error-temperature characteristic. It can be seen from FIG. 19 that, although the ratio error is in minimum at the temperature of about 35° C., if the temperature is decreased or increased from 35° C., the change width of the ratio error is non-linearly increased, so that the measured value of the to-be-measured current of the current sensor is changed due to the temperature characteristic of the Faraday rotator 129.
Therefore, as means for suppressing the above-described problems occurring in the optical fibre 104, a method of not using the Faraday rotator and optically connecting a polarization plane rotation mirror having a wavelength plate to the other end of the optical fibre 104 instead of the mirror 106 is considered. Patent Literature 3 discloses an example of the polarization plane rotation mirror having a λ/4 wavelength plate.
FIG. 17 illustrates a configuration of a polarization plane rotation mirror disclosed in Patent Literature 3. In a polarization plane rotation mirror 116, if a light beam is emitted from a light incidence/emission end surface 117a of an optical fibre 117 and is incident on a first birefringent element 118, the light beam is divided into two linearly polarized light beams of an ordinary ray, or beam and an extraordinary ray, or beam of which polarization directions are perpendicular to each other. Next, the two linearly polarized light beams are incident on a second birefringent element 119. Since directions of the crystal axes of the optical planes of the first birefringent element 118 and a second birefringent element 119 are set to be different from each other by 90 degrees, the light beam transmitting through the first birefringent element 118 as the ordinary ray becomes an extraordinary ray in the second birefringent element 119 and is shifted in the x axis direction of FIG. 17. Therefore, in the case where the two linearly polarized light beams transmit through the first birefringent element 118 and the second birefringent element 119, the two linearly polarized light beams necessarily take optical paths of both of the ordinary ray and the extraordinary ray, and if the first birefringent element 118 and the second birefringent element 119 have the same direction of the crystal axis and the same thickness, the optical path lengths are the same. Since the two polarized components of the ordinary ray and the extraordinary ray are shifted by the same distance by the two birefringent elements 118 and 119, the optical path length difference between the two light beams generated during the division of the first birefringent element 118 is solved before the light beams are reflected by the mirror 122.
Next, the two linearly polarized light beams are incident on the λ/4 wavelength plate 120 so as to be converted into two circularly polarized light beams of which rotation directions of the distal ends of the electric vectors are different. The two circularly polarized light beams emitted from the λ/4 wavelength plate 120 are condensed by a lens 121 and are reflected at one point R on a surface of a mirror 122 in point symmetry; and the optical paths of the circularly polarized light beams are exchanged before and after the reflection; and the rotation directions of the circularly polarized light beams are reversed due to the reflection.
The reflected circularly polarized light beams transmit through the λ/4 wavelength plate 120 again so as to be converted into two linearly polarized light beams of which vibration directions of the electric vectors are different from each other by 90 degrees. At this time, the linearly polarized light beams in the x and y directions in the optical path (forward path) before the reflection becomes the linearly polarized light beams in the y and x directions in the optical path (backward path) after the reflection, respectively. The two linearly polarized light beams re-transmit through the second birefringent element 119 and the first birefringent element 118 and are re-combined as one light beam. The light beam formed through the re-combination is incident on the optical fibre 117.
Since the two light beams are shifted by the same distance by the two birefringent elements 118 and 119 after the reflection by the mirror 122 before the incidence on the optical fibre 117, the optical path length difference between the two light beams which are reflected by the mirror 122 is solved before the re-combined light beam is incident on the optical fibre 117.
In this manner, according to the polarization plane rotation mirror 116, with respect to an arbitrary polarized light beam emitted from the optical fibre 104, the polarization principal axis is rotated by 90 degrees, and in the case where an elliptically polarized component exists, the component is changed into a polarized light beam having the reverse rotation direction, that is, a polarized light beam located at an antipodal point on the Poincare sphere, i.e. a point directly opposite to the other around a circle on the sphere, to be incident on the optical fibre 104, so that the birefringence occurring due to the optical fibre 104 is compensated for, and stabilized measurement of the current sensor 100 can be performed.